Process for hydrodimerizing olefinic compounds

ABSTRACT

IN A PROCESS FOR HYDRODIMERIZING AN OLEFINIC NITRILE, AMIDE OR ESTER BY ELECTROLYZING AN AQUEOUS SOLUTION OF THE OLEFINIC COMPOUND, AN ALKALI METAL SALT AND QUATERNARY AMMONIUM CATIONS, THE SELECTIVELY WITH WHICH THE HYDRODIMER IS PRODUCED IS SURPRISINGLY HIGH WHEN THE SOLUTION CONTAINS LESS THAN ABOUT 5% BY WEIGHT OF THE OLEFINIC COMPOUND, MORE THAN 5% BY WEIGHT OF THE ALKALI METAL SALT AND/OR ALKALI METAL CATIONS CONSTITUTING MORE THAN HALF OF THE TOTAL WEIGHT OF ALL CATIONS IN THE SOLUTION AND THE SOLUTION IS ELECTROLYZED IN CONTACT WITH A CATHODE CONSISTING ESSENTIALLY OF CADMIUM. EVEN IN A CELL IN WHICH THE ANODE IS IN CONTACT WITH THE SLUTION, FOULING OF SUCH A CATHODE PROCEEDS VERY SLOWLY AND THE HYDRODIMER SELECTIVITY REMAINS HIGH FOR AN EXCEPTIONALLY LONG TIME WHEN THE CATHODIC SURFACE HAS A CENTERLINE AVERAGE NOT GREATER THAN ABOUT 90 MICROINCHES.

United States Patent Oflice 3,830,712 Patented Aug. 20, 1974 3,830,712 PROCESS FOR HYDRODIMERIZING OLEFINIC COMPOUNDS Charles R. Campbell and Donald E. Danly, Pensacola, Fla., and Werner H. Mueller, Kelkheim Hornau, Taunus, Germany, assignors to Monsanto Company, St. Louis, M0. N Drawing. Filed Aug. 28, 1972, Ser. No. 284,373 Int. Cl. B01k 3/06; C07b 29/06; CO7c 121/26 U.S. Cl. 204-73 A 16 Claims ABSTRACT OF THE DISCLOSURE In a process for hydrodimerizing an olefinic nitrile, amide or ester by electrolyzing an aqueous solution of the olefinic compound, an alkali metal salt and quaternary ammonium cations, the selectivity with which the hydrodimer is produced is surprisingly high when the solution contains less than about by weight of the olefinic compound, more than 5% by weight of the alkali metal salt and/or alkali metal cations constituting more than half of the total weight of all cations in the solution and the solution is electrolyzed in contact with a cathode consisting essentially of cadmium. Even in a cell in which the anode is in contact with the solution, fouling of such a cathode proceeds very slowly and the hydrodimer selectivity remains high for an exceptionally long time when the cathodic surface has a centerline average not greater than about 90 microinches.

BACKGROUND OF THE INVENTION Production of paraffinic dinitriles, dicarboxamides or dicarboxylates by electrolytic hydroimerization of an alpha,beta-olefinic nitrile, carboxamide or carboxylate is well known, e.g. from U.S. Pat. Nos. 3,193,481 through 3,193,483 which issued to Manuel M. Baizer on July 6, 1965. Although the process has been sufficiently attractive that it has been in commercial use for over seven years, efforts to develop improvements thereon have been continued with particular emphasis on lowering electric power costs and mitigating electrode corrosion and fouling tendencies because of which it has been heretofore commercially preferable to carry out the process with a celldividing membrane. With the objectives of maintaining high electrolyte conductivity while employing a relatively low proportion of organic salts in the electrolysis medium, one approach to improvement of the process has been to carry out the electrolysis in an aqueous solution of a mixture of quaternary ammonium and alkali metal salts together with the olefinic compound to be hydrodimerized.

An example of a process utilizing such an approach is described in Netherlands Patent Application 66,10378 which was laid open for public inspection on Jan. 24, 1967. As described in that application, adiponitrile is produced by electrolyzing a neutral aqueous solution of acrylonitrile, an alkali metal salt of a polyvalent acid such as phosphoric, boric or sulfuric and a small quantity of a quaternary ammonium salt. According to the examples of that application, good selectivities can be achieved when such a process is carried out in an undivided (membraneless) cell having a graphite cathode. As is known in the art, however, commercial-scale use of graphite cathodes in a process of the type discussed herein is not very attractive, primarily because graphite is quite brittle and, at the desirably-elevated hydrodimerization temperatures and generally-optimum electrolyte flow rates of at least several feet per second, sufficiently subject to erosion and/ or fouling that it soon becomes roughened and the selectivity of the reaction (which takes place at the cathode) drops sharply.

superficially, it might have seemed that various other materials having high hydrogen overvoltages could be satisfactorily substituted for graphite as the cathode in electrolytic hydrodimerization (EHD) processes similar to that of Netherlands Application 66,10378 and, in fact, the suitability of a variety of such other materials for use in certain EHD processes has been suggested in the afore cited U.S. Pat. Nos. 3,l93,481-483, in U.S. Pat. N0.

3,511,765 which issued to Fritz Beck et al. on May 12,

1970 and in U.S. Pat. No. 3,595,764 which issued to Maomi Seko et al. on July 27, 1971. However, and notwithstanding those suggestions, it has long been recognized in the art that at least when an olefinic compound EHD medium contains significant amounts of alkali metal salts; the selectivity with which the desired hydrodimer is produced is highly dependent on the specific cathode material employed. To illustrate, in British Pat. No. 1,014,428 which issued to Ivan L. Knounjants et al. on Dec. 22, 1965, the patentees demonstrated that the hydrodimer selectivity in electrolytic hydrodimerization of an olefinic compound such as acrylonitrile is quite high (70-80%) when a graphite cathode is employed with a low-temperature (below 0 C.) aqueous electrolyte containing alkali metal ions in substantial concentration (0.7 to 1.2N) but that with the same electrolyte, an iron cathode yielded only about 20% of the dinitrile (based on the converted monomer) and a cadmium cathode provided practically no reaction product other than the saturated monomer (propionitrile). The high ratios of propionitrile to adiponitrile obtained when various cathode materials other than graphite are employed with an electrolyte containing alkali metal ions are also demonstrated and an explanation is provided by Baizer in Journal of the Electrochemical Society, Vol. 111, No.2, at pages 215-22 (1964).

For reasons including those set forth hereinbefore, a process by which an olefinic nitrile, carboxamide or carboxylate can be electrolytically hydrodimerized with high selectivity while using as the electrolysis medium an aqueous solution containing an alkali metal salt in significant amount and in which the cathode is dimensionally stable and resistant to corrosion for long periods of time is highly attractive for commercial use. Accordingly, the provision of such a process is a primary objective of the invention described herein. Another object of this invention is to provide such a process which can be satisfactorily carried out in an electrolytic cell in which the anode is in contact with the electrolysis medium and despite the anode being subject to corrosion under those circumstances. Further objectives of the invention will be apparent from the following description and Examples in which all percentages are by weight except where otherwise noted.

SUMMARY OF THE INVENTION It has now been discovered that an olefinic compound having the formula R C=CR-X wherein X is CN, CONR or COOR', R is hydrogen or R and R is C -C alkyl can be hydrodimerized to prepare a hydrodimer having the formula XCHRCR CR CHRX wherein X and R have the aforesaid significance with a high molar selectivity (at least about and in many cases at least about based on the converted olefinic compound by electrolyzing an aqueous solution of the olefinic compound, quaternary ammonium cations and an alkali metal salt in contact with a cathodic surface consisting essentially of cadmium. In one embodiment of the invention, the aqueous solution has dissolved therein at least about 0.1% of the olefinic compound, quaternary ammonium cations in a concentration of at least about 10- gram mol per liter and at least about 0.1% of alkali metal salt sufiicient to provide alkali metal cations constituting more than half of the total weight of all cations in the solution. In another embodiment of the invention, the aqueous solution has dissolved therein at least about 0.1% of the alkali metal salt, quaternary ammonium cations in a concentration of at least about 10 gram mol per liter and at least about 0.1% but less than about 5% of the olefinic compound. In still another embodiment of the invention, the aqueous solution has dissolved therein at least about 0.1% of the olefinic compound, quaternary ammonium cations in a concentration of at least about l gram mol per liter and at least about 5% of the alkali metal salt. Even when the process is carried out in an electrolytic cell in which the anode is in contact with the solution, fouling of the cathode proceeds very slowly and the hydrodimer selectivity remains high for an exceptionally long time when the cathodic surface has a centerline average not greater than about 90 microinches. Each of the embodiments of the invention is particularly useful in the preparation of adiponitrile, a nylon 66 intermediate, by the hydrodimerization of acrylonitrile.

DETAILED DESCRIPTION OF THE INVENTION Olefinic compounds that can be hydrodimerized by the process of this invention include those having the structural formula R C=CRX wherein X is --CN, CONR or -COOR', R is hydrogen or R and R is C C alkyl (i.e., methyl, ethyl, n-propyl, isopropyl, nbutyl, isobutyl or tertbutyl). Compounds having that formula are known as having alpha, beta mono-unsaturation and in each such compound, at least one R may be R while at least one other R is hydrogen and at least one R, if present, may be an alkyl group containing a given number of carbon atoms while at least one other R, if present, is an alkyl group containing a different number of carbon atoms. Such compounds include olefinic nitriles such as, for example, acrylonitrile, methacrylonitrile, crotononitrile, 2 methylenebutyronitrile, 2 pentenenitrile, 2 methylenevaleronitrile, 2 methylenehexanenitrile, tiglonitrile or 2 ethylidenehexanenitrile; olefinic carboxylates such as, for example, methyl acrylate, ethyl acrylate or ethyl crotonate; and olefinic carboxamides such as, for example, acrylamide, methacrylamide, N,N- diethylacrylamide or N,N-diethylcrotonamide. Products of hydrodimerization of such compounds have the structural formula X-CHRCR CH CHRX wherein X and R have the aforesaid significance, i.e., paraffinic dinitriles such as, for example, adiponitrile and 2,5 dimethyladiponitrile; parafiinic dicarboxylates such as, for example, dimethyladipate and diethyl 3,4 dimethyladipate; and paraflinic dicarboxarnides such as, for example, adipamide, dimethyladipamide and N,N' dimethyl 2,5- dimethyladipamide. All of such hydrodimers are useful in the manufacture of high molecular weight condensation polymers, e.g. by reaction with dihydroxy or dicarboxylic compounds, and in the case of the dinitriles, as intermediates which can be hydrogenated by known processes to prepare paraffinic diamines that are similarly useful in the manufacture of high molecular weight condensation polymers. Other examples of the various olefinic compounds that can be hydrodimerized by the process of this invention and the hydrodimers thereby produced are identified in the aforecited US. Pat. Nos. 3,193,48l-483.

The invention is herein described in terms of electrolyzing an aqueous solution having dissolved therein certain proportions of the olefinic compound to be hydrodimerized, quaternary ammonium cations and an alkali metal salt. This is intended to indicate that the electrolysis medium can be a single-phase aqueous solution containing essentially no undissolved organic phase, by which is meant that the solution may contain no undissolved organic phase or, alternatively, a minute proportion of undissolved organic phase, such as might remain entrained in the aqueous solution despite the latter being permitted to stand without agitation after electrolysis and cooling to separate a product-containing organic phase, but the presence of which does not have any significant effect on the olefinic compound conversion per pass or hydrodimer selectivity achieved when the separated aqueous phase is recycled for further electrolysis in accordance with the process of this invention. Such a minute proportion, if present, would be generally less than 5% (e.g. not more than 4.5%) of the aqueous solution, more typically less than about 2% (e.g. not more than 1.8%) of the aqueous solution and preferably less than 1% (e.g. not more than 0.8%) of the aqueous solution. In another embodiment, the invention can be carried out by electrolyzing an aqueous solution of the type described hereinbefore but having dispersed therein an undissolved organic phase in a larger proportion (e.g. from about 5% up to 20% or even 50% or more of the aqueous solution) which may or may not significantly affect the conversion per pass or hydrodimer selectivity depending on other conditions of the process. In continuous process embodiments involving recycle of unconverted olefinic compound and whether present in a minute or larger proportion, such an organic phase would be normally made up mainly of the olefinic compound to be hydrodimerized and the hydrodimer product with some small amounts of organic hydrodimerization byproducts, quaternary ammonium cations, etc. possibly also present. In any event, however, the concentrations of the recited constituents of the aqueous solution to be electrolyzed, as set forth in this specification and the appended claims, are with reference to the recited aqueous solution alone and not the combined contents of said aqueous solution and an undissolved organic phase which, as aforesaid, may be present in the aqueous solution as the process of this invention is carried out.

Referring now to the constituents of the aqueous phase, the olefinic compound to be hydrodimerized will be present in at least such a proportion that electrolysis of the solution, as described herein, will result in a substantial amount of the desired hydrodimer being produced. That proportion is generally at least about 0.1% of the aqueous solution, more typically at least about 0.5% of the aqueous solution and, in some embodiments of the invention, preferably at least about 1% of the aqueous solution. Inclusion of one or more additional constituents which increase the solubility of the olefinic compound in the solution may permit the carrying out of the process with the solution containing relatively high proportions of the olefinic compound, e.g. at least about 5% or even 10% or more, but in many embodiments of the invention, the aqueous solution contains less than about 5% (e.g. not more than 4.5%) of the olefinic compound and, in most of those embodiments, preferably not more than about 1.8% of the olefinic compound.

The minimum required, proportion of quaternary ammonium cations is very small. In general, there need be only an amount sufficient to provide the desired hydrodimer selectivity (e.g. at least about although much higher proportions can be present if desired or convenient. In most cases, the quaternary ammonium cations are present in a concentration of at least about 10- gram mol per liter of the aqueous solution. Even more typically their concentration is at least about 10" gram mol per liter of the solution and, in many embodiments, preferably at least about 10* gram mol per liter. Although higher proportions may be present in some cases, as aforesaid, the quaternary ammonium cations are generally present in the aqueous solution in a concentration lower than about 0.5 gram mol per liter and even more usually, in a concentration not higher than about 0.1 gram mol per liter. In some preferred embodiments, the concentration of quaternary ammonium cations in the solution is at least about 2 10 gram mol per liter but not more than about 5 1() and, in many cases, not more than about 2 10 gram mol per liter.

The quaternary ammonium cations that are present in such concentrations are those positively-charged ions in which a nitrogen atom has a valence of five and is directly linked to other atoms (e.g. carbon) satisfying four fifths of that valence. Such cations may be cyclic, as in the case of the piperidiniums, pyrrolidiniums and morpholiniums, but they are generally of the type in which the nitrogen atom is directly linked to a total of four monovalent organic groups from the group consisting of alkyl or aryl radicals or combinations thereof. The aryl groups contain typically from six to twelve carbon atoms and preferably only one aromatic ring as in, for example, a phenyl or benzyl radical. The alkyl groups can be straight-chain, branched or cyclic and each typically contains from one to twelve carbon atoms. Although quaternary ammonium cations containing a combination of such alkyl and aryl groups (e.g. benzyltriethylammonium ions) can be used, many embodiments of the invention are preferably carried out with tetraalkylammonium ions and superior results are generally obtained with the use of those containing at least three C -C alkyl groups and a total of from 8 to 24 carbon atoms in the four alkyl groups, e.g. tetraethyl-, ethyltripropyl-, ethyltributyl-, ethyltriamyl-, ethyltrihexyl-, octyltriethyl-, tetrapropyl-, methyltripropyl-, decyltripropyl-, methyltributyl-, tetrabutyl-, amyltributyl-, tetraamyl-, tetrahexyl-, ethyltrihexyl-, diethyldioctylammonium and many others referred to in the aforecited US. Pat. Nos. 3,193,481-483. Most practical from the economic standpoint are generally those tetraalkylammonium ions in which each alkyl group contains two to five carbon atoms, e.g. diethyldiamyl-, tetrapropyl-, tetrabutyl-, amyltripropyl-, tetraamylammonium, etc. Such cations can be incorporated into the aqueous solution to be electrolyzed in any convenient manner, e.g. by dissolving the quaternary ammonium hydroxide or a salt thereof in the solution in the amount required to provide the desired quarternary ammonium cation concentration.

The alkali metal salts which can be employed in the invention are those of sodium, potassium, lithium, cesium and rubidium. Generally preferred for economic reasons are those of lithium and especially sodium and potassium. They may be salts of a monovalent acid, e.g. a perchlorate, a nitrate or a halide such as a chloride or bromide. In some cases, e.g. where corrosion control is more of a factor, it may be desirable to use an alkali metal salt of a polyvalent acid, e.g. an orthophosphate, borate, carbonate or sulfate, and particularly an incompletely-substituted salt of that type, i.e. a salt in which the anion has at least one valence thereof satisfied by hydrogen and at least one other valence thereof satisfied by an alkali metal. Examples of such salts include disodium phosphate (Na HPO potassium acid phosphate (KH PO sodium bicarbonate (NaHCO dipotassium borate (K HBO and sodium acid sulfate (NaHSO Also useful are the alkali metal salts of condensed acids such as pyrophosphoric, metaphosphoric, metaboric, pyroboric and the like (e.g. sodium pyrophosphate, potassium metaborate, etc.). Depending on the acidity of the aqueous solution to be electrolyzed, the stoichiometric proportions of such anions and alkali metal cations in the solution may correspond to a mixture of two or more of such salts, e.g. a mixture of sodium acid phosphate and disodium phosphate, and such a mixture of salts is intended to be within the scope of the expression alkali metal salt as used in this specification and the appended claims. Any of the alkali metal salts may be dissolved in the aqueous solution as such or otherwise, e.g. as the alkali metal hydroxide and the acid necessary to neutralize the hydroxide to the extent of the desired acidity of the aqueous solution.

The concentration of alkali metal salt in the solution should be at least sufiicient to substantially increase the electrical conductivity of the solution above its conductivity without such a salt being present. In general, there is also enough alkali metal salt dissolved in the solution to provide alkali metal cations constituting more than half of the total weight of all cations in the solution. In most cases, the solution has dissolved therein at least about 0.1% of the alkali metal salt. More advantageous conductivity levels are achieved when the solution has dissolved therein at least about 1% of alkali metal salt or, more preferably, at least about 2% of such a salt. In many cases, optimum process conditions include the solution having dissolved therein more than 5% (typically at least 5.5%) of alkali metal salt. The maximum amount of alkali metal salt in the solution is limited only by its solubility therein, which varies with the particular salt employed. With salts such as sodium or potassium phosphates, it is generally most convenient when the solution contains between about 8% and about 12% of such a salt or mixture thereof.

The acidity of the solution need only be such that a neutral or alkaline condition prevails at the cathode. Since there is normally an acidity gradient across the cell, pH at the anode can be lower than seven, if desired. In most cases, however, pH of the overall solution should be at least about two, is preferably at least about five and when the solution is in contact with certain metals subject to corrosion, is most conveniently at least about seven. Also in most cases, the overall solution pH is not higher than about twelve, typically not higher than about eleven and, with the use of sodium and/or potassium phosphates, generally not higher than about nine.

The temperature of the solution may be at any level compatible with existence as such of the solution itself, i.e., above its freezing point but below its boiling point under the pressure employed. Good results can be achieved between about 5 and about 75 C. or at even higher temperatures if pressures substantially above one atmosphere are employed. The optimum temperature range will vary with the specific olefinic compound and hydrodimer, among other factors, but in hydrodimerization of acrylonitrile to adiponitrile, an electrolysis temperature between about 25 and about 65 C. is usually preferred.

Although not necessary, a liquid-impermeable cathode is usually preferred. With the use of such a cathode, the aqueous solution to be electrolyzed is generally passed along the surface thereof at a linear velocity with reference to the adjacent cathodic surface of at least about one foot per second, preferably at least about two feet per second and even more preferably between about three and about eight feet per second although, if desired, a solution velocity up to twenty feet per second or higher can be employed, if desired. The gap between the anode and cathode can be very narrow, e.g. about 40 mils or less, or as wide as one-half inch or even wider, but is generally of a width between about 60 mils and about onequarter inch.

As is well-known, electrolytic hydrodimerization of an olefinic compound having a formula as set forth hereinbefore must be carried out in contact with a cathodic surface having a cathode potential sufiicient for hydrodimerization of that compound. In general, there is no minimum current density with which the process can be carried out at such a cathodic surface but in most cases, a current density of at least about 0.01 amp per square centimeter of the cathodic surface is used and a current density of at least about 0.05 amp per square centimeter of the cathodic surface is usually preferred. Although higher current densities may be practical in some instances, those generally employed in the present process are not higher than about 1.5 amps per square centimeter and even more typically not higher than about 0.75 amp per square centimeter of the aforedescribed cathodic surface. Depending on other process variables, current densities not higher than about 0.5 amp per square centimeter may be preferred in some embodiments of the invention.

As aforesaid, the process of this invention is carried out with a cathodic surface consisting essentially of cadmium, meaning that the cathodic surface contains a very high percentage of cadmium (generally at least about more typically at least about and preferably at least about 98%) but that it may contain a small amount of one or more other constituents that do not alter the nature of the cadmium cathode so as to prevent it from providing the advantages of the present invention, particularly as described herein. Such other constituents, if present, are desirably other materials having relatively high hydrogen overvoltages, e.g. thallium, mercury, manganese, lead, zinc, tin, graphite, etc., but preferably not such materials of relatively low hydrogen overvoltage as copper or nickel in any concentration higher than about 0.05% or, even more desirably, about 0.02%, based on the cadmium in the cathodic surface. When such other materials are present in a relatively high concentration such as, for example, from about 0.5% up to about or higher, they are preferably lead and/or mercury. However, best results are generally obtained when the cathodic surface has a cadmium content of at least about 99.5%, even more typically at least about 99.8% and most desirably at least about 99.9% as in ASTM Designation B440-66T (issued 1966). Such proportions are expressed, of course, without reference to any constituents of the aqueous solution to be electrolyzed although, in operation of the process, certain of those constituents may become associated with the cathodic surface, either transiently or otherwise so as to act as a part of the cathodic surface in the sense of having the potential difference between the solution and cathode required for hydrodimerization of the olefinic starting material.

Cathodes employed in this invention can be prepared by any of various techniques such as, for example, electroplating of cadmium on any suitably-shaped substrate of some other material, e.g. a metal having greater structural rigidity, or by chemically, thermally and/or mechanically bonding a layer of cadmium or an alloy thereof containing one or more of the aforementioned other optionallypresent cathode constituents to a similar substrate. Alternatively, a plate, sheet, rod or any other suitable configuration consisting essentially of cadmium may be used without such a substrate, if desired.

As aforesaid and contrary to expectations based on the disclosure of British Pat. No. 1,014,428, use of the process embodiments described herein including a cathodic surface consisting essentially of cadmium provides the desired hydrodimer with a high molar selectivity, based on the converted olefinic starting material, and for clearly long enough periods of time for attractive commercial practice of the process. The hydrodimer selectivity of the present invention, as contrasted with essentially zero in the British Pat. No. 1,014,428 process embodiments using a cadmium cathode, is normally at least about 75 i.e., at least about 75% of the mols of converted olefinic starting material are converted to the desired dinitrile, dicarboxylate or dicarboxamide. In many cases, the molar selectivity of the present process is at least about 80% and, in some instances including certain embodiments employed in hydrodimerization of acrylonitrile to adiponitrile, as high as 85% or even higher.

The process of this invention can be satisfactorily carried out in a divided cell having a cation-permeable membrane, diaphragm or the like separating the anode and cathode compartments of the cell in such a way that the aqueous solution undergoing electrolysis is not in contact with the anode of the cell and products of anode corrosion, if any, are substantially prevented from migrating to the cathode of the cell. The process can also be carried out in a cell that is not divided in that manner, i.e., in an electrolytic cell in which the aforedescribed aqueous solution is in contact with the anode of the cell while it is in contact with the cathode of the same cell, and in which the anode is composed of a material not corroded by the solution at a substantial rate (e.g. at least about 10 inch per year) such as, for example, one of the materials conventionally regarded as corrosion-proof (e.g. platinum, various alloys of platinum, other precious metals and alloys thereof, lead dioxide, carbon, etc.). In both of such embodiments, anode corrosion products normally do not reach the cathode of the cell in a quantity large enough to plate out on or foul the cathode to a degree sulficient to greatly lower the hydrodimer selectivity of the process and it has been found that the surface smoothness of the cathode is generally not of critical importance to longterm maintenance of high selectivities when that is the case.

In another embodiment, the process of this invention can be carried out in an undivided cell in which the anode is in contact with the aqueous solution, as aforesaid, and the anode is composed of a material which, depending on process conditions such as the particular alkali metal salt employed, the solution temperature, etc., may or may not be corroded by the solution at a substantial rate under the electrolysis conditions. Such less corrosion-resistant anode materials include the ferrous metals such as iron and steel, magnetite, nickel, nickel silicide and, in fact, any metal or alloy capable of being passivated, particularly if the solution undergoing electrolysis is alkaline or at least not strongly acidic (i.e., pH not substantially below seven). When the process is carried out with an anode comprising such a less corrosion-resistant material in contact with the solution undergoing electrolysis and the anode is substantially corroded under the conditions of the process, e.g. such that products of corrosion of the anode become dispersed in the electrolysis medium and subsequently tend to plate out on and/or foul the cathodic surface to a degree which would otherwise substantially lower the hydrodimerization selectivity, it is generally most advantageous for long-term maintenance of high hydrodimer selectivities to inhibit such plating out and/ or fouling by employing a cathodic surface having a degree of smoothness corresponding to a centerline average not greater than about microinches as determined in accordance with the definition of centerline average set forth in American Standard ASA B46.11962 (Surface Texture) published by The American Society of Mechanical Engineers, 345 East 47th Street, New York, NY. In most cases, the centerline average of the cathodic surface employed in this embodiment of the present process is desirably less than about 70 microinches, preferably less than about 50 microinches and, for superior results in many cases, less than about 30 microinches, all determined in accordance with the definition in the aforecited ASA publication. Centerline average, as the term is used herein, can be measured by various procedures and types of apparatus, exemplary of which are the Rank Taylor Hobson Talysurf 4 and the procedures described in the Talysurf 4 Operators Handbook distributed by Rank Precision Industries Ltd., Metrology Division, PO. Box 36, Leicester House, Lee Circle, Leicester LE1 9JB, England and in the USA. by Engis Equipment Company, 8035 Austin Avenue, Morton Grove, Ill. 60053. In some embodiments in which anode corrosion may otherwise proceed at a relatively high rate, it may be desirable to also include in the electrolysis medium a small amount (generally between about 0.02% and about 2%) of an inhibitor of corrosion of the anode material employed (e.g. an alkali metal salt of a condensed acid such as pyrophosphoric, metaboric or the like when the anode material comprises a ferrous metal) and/or a similarly small amount of a chelating agent for the anode metal (e.g. a diacetic or polyacetic acid compound such as ethylenediaminetetraacetic acid, nitrilotriacetic acid or the like).

The following specific examples of the process of this invention are included for purposes of illustration only and do not imply any limitations on the scope of the mvention.

Example I In a continuous process, an aqueous solution having dissolved therein approximately 1.6% acrylonitrile, 1.2% adiponitrile, 0.2% acrylonitrile EHD byproducts, 5.8 10 gram mol per liter of ethyltributylammonium cations, 10% of a mixture of incompletely-substituted sodium orthophosphates corresponding to the solution pH of 9 (approximately Na H PO 0.1% of a ferrous 9 metal corrosion inhibitor (sodium pyrophosphate) and 0.05% of ethylenediaminetetraacetic acid was circulated at 55 C. and-a velocity between 4 and 4.5 feet per second through an undivided electrolytic cell having a steel anode separated by a gap of 107 mils from a cathodic surface composed of a rolled sheet of cadmium conforming to ASTM Designation B440-66T issued 1966 (at least 99.9% Cd) and having a centerline average of about microinches measured in accordance with the definition set forth in American Standard ASA B46.11962. The solution, which also had entrained therein less than 1% by weight of an organic phase containing about 54% adiponitrile, 29% acrylonitrile, 9% acrylonitrile EHD byproducts and 8% water, was electrolyzed as it passed through the cell with a voltage drop across the cell of 4.7 volts and a current density of 0.27 amp per square centimeter of cathodic surface and then fed into a decanter for equilibration with an accumulated upper layer having approxi-- mately the composition of the aforedescribed organic phase and withdrawal of equilibrated lower (aqueous) layer for recycle through the cell. After 776 hours of electrolysis during which acrylonitrile and water were continuously added to the circulating aqueous solution and equivalent amount of product was removed from the decanter upper layer, it was found that acrylonitrile in the solution had been converted to adiponitrile with an average molar selectivity of 86.1% and the cathodic surface had corroded at the average rate of only 0.003 inches per year.

Example H In a continuous process, an aqueous solution having dissolved therein approximately 1.6% acrylonitrile, 1.2% adiponitrile, 0.2% acrylonitrile EHD byproducts, ethyltributylammonium cations in a concentration varying between 9 and 25 10- gram mol per liter, 9% of a mixture of incompletely-substituted sodium orthophosphates corresponding to the solution pH of 9 (approximately Na H POQ, 0.1% of a ferrous metal corrosion inhibitor (sodium pyrophosphate) and 0.05% of ethylenediaminetetraacetic acid was circulated at a temperature between 50 and 55 C. and a velocity between three and four feet per second through an undivided electrolytic cell having a steel anode separated by a gap of 125 mils from a cathodic surface composed of a rolled sheet of cadmium essentially the same in composition and centerline average as that employed in Example I. The solution, which also had entrained therein less than 4% by weight of an organic phase containing about 54% adiponitrile, 29% acrylonitrile, 9% acrylonitrile EHD byproducts and 8% water, was electrolyzed as it passed through the cell with a voltage drop across the cell of 4.5 volts and a current density of 0.23 amp per square centimeter of cathodic surface and then fed into a decanter for equilibration with an accumulated upper layer having approximately the composition of the aforedescribed organic phase and then withdrawal of equilibrated lower (aqueous) layer for recycle through the cell. After 325 hours of electrolysis during which acrylonitrile and water were continuously added to the circulating aqueous solution and an equivalent amount of product was removed from the decanter upper layer, it was found that acrylonitrile in the solution had been converted to adiponitrile with an average molar selectivity of 86.1% and the cathodic surface had corroded at the average rate of only 0.002 inches per year.

[Example III In a continuous process, an aqueous solution having dissolved therein approximately 0.8% acrylonitrile, 1.1% adiponitrile, 0.15% acrylonitrile EHD byproducts, 8X10 gram mol per liter of tetrabutylammonium cations, 13% of a mixture of incompletely-substituted sodium orthophosphates corresponding to the solution pH of 8 (approximately Na H PO and 0.05-0.1% of a ferrous metal corrosion inhibitor (sodium pyrophosphate) was circulated at 50 C. and a velocity of four feet per second through an undivided electrolytic cell having a steel anode separated by a gap of 94 mils from a cathodic surface composed of a rolled sheet of cadmium essentially the same in composition and centerline average as that employed in Examples I and II. The solution, which also had entrained therein less than 4% by weight of an organic phase containing about 64% adiponitrile, 17% acrylonitrile, 11% acrylonitrile EHD byproducts and 8% water, was electrolyzed as it passed through the cell with a voltage drop across the cell of 4.35 volts and a current density of 0.25 amp per square centimeter of cathodic surface and then fed into a decanter for equilibration with an accumulated upper layer having approximately the composition of the aforedescribed organic phase and then withdrawal of equilibrated lower (aqueous) layer for recycle through the cell. After 28 hours of electrolysis during which acrylonitrile and water were continuously added to the circulating aqueous solution and an equivalent amount of product was removed fromthe decanter upper layer, it was found that acrylonitrile in the solution had been converted to adiponitrile with an average molar selectivity of 85.8% and corrosion of the cathodic surface had been negligible, i.e., less than 0.001 inch per year.

We claim:

1. A process for hydrodimerizing an olefinic compound having the formula R C=CR-X wherein --X is CN, CONR or COOR, R is hydrogen or R and R is C -C alkyl which comprises electrolyzing an aqueous solution having dissolved therein at least about 0.5% but less than about 5% by weight of said olefinic compound, at least about 1% by weight of sodium or potassium salt selected from the group consisting of phosphate, borate, perchlorate, carbonate and sulfate sufficient to provide sodium or potassium ions constituting more than half of the total weight of all cations in the solution and from about 10* to about 0.5 gram mol per liter of C -C tetraalkylammonium ions containing at least three C C alkyl groups in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of said olefinic compound and consisting essentially of cadmium with a current density between about 0.01 and about 1.5 amps per square centimeter of said cathodic s7lgr fae and at a temperature between about 5 and about 2. The process of Claim 1, said solution having dissolved therein not more than about 1.8% by weight of said olefinic compound.

3. The process of Claim 1, said solution having dissolved therein more than 5% by weight of the sodium or potassium salt.

4. The process of Claim 1 wherein the olefinic compound is acrylonitrile, said solution having dissolved therein at least about 10- gram mol per liter of the C C- tetraalkylammonium ions.

5. The process of Claim 4, said solution having dissolved therein not more than about 1.8% by weight of said olefinic compound.

6. The process of Claim 4, said solution having dissolved therein more than 5% by weight of the sodium or potassium salt.

7. A process for hydrodimerizing an olefinic compound having the formula R C=CR-X wherein X is -CN, --CONR or COOR', R is hydrogen or R and R is C -C alkyl which comprises electrolyzing an aqueous solution having dissolved therein at least about 0.5% but less than about 5% by weight of said olefinic compound, at least about 1% by weight of sodium or potas- SlUIIl salt selected from the group consisting of phosphate, borate, perchlorate, carbonate and sulfate sufficient to provide sodium or potassium ions constituting more than half of the total Weight of all cations in the solution and from about 10- to about 0.5 gram mol per liter of Cry-C24 tetraalkylammonium ions containing at least three C -C alkyl groups in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of said olefinic compound and consisting essentially of cadmium with a current density between about 0.01 and about 1.5 amps per square centimeter of said cathodic surface and at a temperature between about and about 75 C. in an electrolytic cell in which the anode is in contact with the solution and said cathodic surface has a centerline average not greater than about 90 microinches.

8. The process of Claim 7, wherein the anode comprises a ferrous metal and the centerline average of the cathodic surface is less than about 50 microinches.

9. The process of Claim 7 wherein the olefinic compound is acrylonitrile, said solution having dissolved therein at least about 10- gram mol per liter of the C C tetraalkylammonium ions.

10. The process of Claim 9, wherein the anode comprises a ferrous metal and the centerline average of the cathodic surface is less than about 50 microinches.

11. A process for hydrodimerizing acrylonitrile which comprises electrolyzing an aqueous solution having dissolved therein from about 0.5 to about 1.8% by weight of acrylonitrile, from about 1% to about 12% by weight of sodium or potassium phosphate sufiicient to provide sodium or potassium ions constituting more than half of the total weight of all cations in the solution and tetra(C -C a1kyl)ammonium ions in a concentration of from about l0 to about 10- mol per liter in contact with a cathodic surface having a cathode potential sufficient for hydrodimerization of acrylonitrile and consisting essentially of cadmium with a current density between 12' about 0.01 and about 0.75 amp per square centimeter of said cathodic surface while passing the solution along said cathodic surface at a velocity of at least about two feet per second, said solution having a pH of at least about 7 and a temperature between about 5 and about C.

12. The process of Claim 11, said solution having dissolved therein more than 5% by weight of alkali metal salt.

13. The process of Claim 11, said solution having dissolved therein between about 10* and 10- gram mol per liter of tetra(C -C alkyl)ammonium ions.

14. The process of Claim 11 carried out in an electro lytic cell wherein the anode is in contact with said solution and said cathodic surface has a centerline average not greater than about microinches.

15. The process of Claim 14, wherein the anode comprises a ferrous metal and the centerline average of the cathodic surface is less than about 50 microinches.

16. The process of Claim 11, wherein the solution has dispersed therein an undissolved organic phase in a proportion up to 50% by weight of said solution.

References Cited UNITED STATES PATENTS 5/1970 Beck et al. 204 73*A 9/1972 Fox et al. 2047 3"A US. Cl. X.R. 204--73 A 

